Passive Films on Iron in Phosphate and Borate Solutions

Abstract

The passive films formed on iron in neutral, alkaline phosphate and alkaline borate solutions were investigated by using electrochemical and ellipsometric techniques. Chemical analyses of iron ion dissolved from the film and ellipsometric measurements of δP-δA curve during galvanostatic cathodic film reduction have shown that the passive films formed in these solutions consist of a deposit layer next to the solution and a barrier layer in contact with the metal. The oxidation state of iron ion in the deposit layer is in the iron (III) state and is scarcely affected by pH and anions in the solution. The barrier layer composition, however, depends on pH and anion species. In borate solutions, the barrier layer composition changes from ferric oxide (pH 8.42) to magnetite-like oxide (pH 11.50) with increase of solution pH. On the other hand, its composition in phosphate solutions contains iron (II) ions giving rise to the mean oxidation valency, Z_Fe=2.33 in the pH range of 8.42 to 12.60. It has been found that depletion or enrichment of iron ions occurs at the deposit/barrier layers interface in borate solutions of pH 8.42 and 11.50 with the degree of depletion or enrichment being dependent on pH. No iron depletion or enrichment, however, is found in the passive films formed in phosphate solutions. The results are explained in terms of the ion permselective property of the passive films formed in phosphate and borate solutions.